EDIT: my fault - had downloaded the Search module from the github page of the original module, rather than that of the improved version by Daniel-KM.

Hi, I’m currently setting up Solr search in v1.2.0, with the latest versions of Search (+ jQueryUI) and Solr modules from their respective github pages. PHP 7.0, Solr 5.5.5. Indexing is working fine. I’ve created a page for the search, following the instructions of the Solr module github page, and added the URL to site navigation. When I go to the url (localhost/[omeka-s installation]/s/[site]/search) and try a search, I get “Omeka S has encountered an error”. Environment is set to developer in .htaccess and all error reporting is enabled in php.ini, but I’m unable to get anything more. Server response is internal server error 500. Anyone has an idea how I could get something more detailed about this issue?

Yes, the original Search/Solr modules weren’t updated since two years and the pull requests are not yet integrated.
Yes, to see the errors, you just have to set SetEnv APPLICATION_ENV "development" in .htaccess, then you can see the logs in your browser or in logs/application.log. There may be some php errors that are logged in the php errors.log somewhere in your server too.
The first thing to check is it there is an error in the Solr page (admin/solr). Then, in the search manager (admin/search-manager) and in the form you have prepared.